On 10/6/10 1:58 PM, kj wrote:
The short version of this question is: where can I find the algorithm used by the tuple class's __hash__ method?
The function tuplehash() in Objects/tupleobject.c, predictably enough.
Now, for the long version of this question, I'm working with some complext Python objects that I want to be able to compare for equality easily. These objects are non-mutable once they are created, so I would like to use a two-step comparison for equality, based on the assumption that I can compute (either at creation time, or as needed and memoized) a hashkey/digest for each object. The test for equality of two of these objects would first compare their hashkeys. If they are different, the two objects are declared different; if they match, then a more stringent test for equality is performed.
The most straightforward way to implement __hash__ for a complicated object is to normalize its relevant data to a tuple of hashable objects and then call hash() on that tuple. A straightforward way to compare such objects is to calculate that very same normalized tuple for each one and compare those tuples. Cache it if necessary. Don't bother hashing to implement __eq__ unless if you are really optimizing for space and time.
-- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco -- http://mail.python.org/mailman/listinfo/python-list
